How to pronounce Mons Mensae? Could you write it in international phonetic alphabet? I think that will be [Mʌns Mensæ]... Is that right?
P.S. I intended the constellation known as Mons Mensae.
Because of Latin's long history of use, there are many traditions for how to pronounce it.
Since this is a site about Latin, I'll start with the Latin pronunciations. The Classical Latin pronunciation was something like /moːns ˈmeːnsa͡e/, phonemically.
Phonetically, there are a few further details that it may be useful to know about.
Many scholars believe that the evidence indicates that at some point in Latin, /n/ before the sounds /s/ or /f/ was not pronounced as the consonant [n], but coalesced with the preceding vowel to form a nasalized long vowel. (Vowels before nf or ns are always long as a rule in Classical Latin.) However, Allen (1978) says that a nasal consonant sound ([n] before [s]; possibly a labiodental [ɱ] before [f]) was subsequently reintroduced into the pronunciation of words that were spelled in Classical Latin with nf and ns (pp. 28-30, p. 66). Other words came to lose the n in spelling, and presumably also in pronunciation. A fair amount of variability seems to have persisted throughout the Classical period and we see reflexes in the Romance languages of forms with loss of n corresponding to Classical Latin forms that Allen indicates would have been pronounced with restored n (e.g. Italian isola corresponding to Classical Latin insula).
The precise quality of /oː/ ("long o") and /eː/ ("long e") is only known approximately, and it probably changed slightly over time. It's clear that they were more or less mid vowels. The evidence seems to point to these vowels being realized in earlier stages of Latin with relatively open qualities such as [o̞ː] and [e̞ː], or even [ɔː] and [ɛː], since in Old Latin there was a vowel written ei, thought to be close-mid [eː], that contrasted with both "long e" and "long i" (by the time of Classical Latin, ei had merged with "long i"). However, in later stages, it seems fairly clear they were realized with a closer quality than the corresponding short vowels [ɔ] and [ɛ], so they are often transcribed [oː] and [eː].
The vowel sound represented by “ae”, which I’ve transcribed phonemically as /a͡e/ (a fairly arbitrary decision), also changed over time. Originally, it was a phonetic diphthong pronounced more or less as [ai̯] or [aɪ̯] (and written as "ai"), which could be phonemically analyzed as a vowel-glide sequence /aj/. The offglide seems to have become more open over time, so [ae̯] was probably used at some point (which would explain the change in spelling to "ae"). In later stages of the language, "ae" became a long monophthong, open-mid [ɛː], which merged with the reflex of short "e" to become *ɛ in Proto-Romance.
So phonetically, "mons mensae" would probably be something like [moːns ˈmeːnsae̯] or [moːns ˈmeːnsɛː].
I've been vague in my description of the dating of these sound changes because
I'm not as familiar with Ecclesiastical pronunciation of Latin (which is mostly Italian-influenced), so I might get something wrong here, but it seems it would be pronounced /mɔns ˈmɛnsɛ/ in this tradition.
The main thing I'm not sure about is the vowels. Some varieties of Italian have a phonemic distinction between close-mid /e o/ and open-mid /ɛ ɔ/ in stressed syllables. However, it seems that there is no such distinction in the Italian pronunciation of Latinate words; or at least, I have not found any agreement about which would be used where (see the talk on this Wiktionary page: Template talk:la-IPA for an example of the confusion).
Most sources I've found seem to indicate that Ecclesiastical Latin only uses open-mid [ɛ ɔ].
However, in Italian at least the distinction is neutralized in unstressed syllables to close-mid [e o], so I don't know if the usual Italian pronunciation would phonetically be [mɔns ˈmɛnse].
The traditional English pronunciation would be /mɒnz mɛnsiː/. I won’t focus too much on describing dialectal and sub-phonemic variations of this since this is a site about Latin, not about English, but the most notable points of variation might be the vowel in “mons” (which would be /ɑ/ in General American English) and a possible epenthetic [t] between /n/ and /s/.
The “restored” English pronunciation, using the usual vowel equivalences described e.g. here (http://www.txclassics.org/old/PronunciationGuide.pdf) would be /moʊns meɪnsaɪ/. However, in English-language contexts, many people use an inconsistent mix of restored pronunciation and more traditional English pronunciation, so you’re probably more likely to hear things like /mɒnz mɛnsaɪ/, /moʊnz mɛnsaɪ/ or /mɔːnz mɛnsaɪ/.
There are several different schemes, used in different fields.
In general English use (common in the sciences), it would be /mɔnz mɛntsej/.
In Reconstructed Classical pronunciation (common in the classics), it would be closer to /mons mensaj/.
In Ecclesiastic pronunciation (common in the Catholic church), something like /mons mɛnse/.
So as you can see, you have more freedom with your vowels. But I don't know of any pronunciation system which would use /ʌ/ or /æ/.